An ink-jet recording apparatus has been known, which has a recording head for discharging ink as droplets onto a recording paper to print an image. The ink-jet recording apparatus is provided with at least an ink tank containing ink, to supply the ink from the ink tank to the recording head. In a serial ink-jet recording apparatus, a recording head is mounted to a carriage that moves back and forth in a main scanning direction, a widthwise direction of a recording paper. Each time the carriage makes one lap, the recording head records a line of image, and the recording paper is fed in a sub scanning direction orthogonal to the main scanning direction by an amount corresponding to the image line. Thus, an image frame is recorded line after line.
Because the ink is a consumable material, the ink tank is often formed as a cartridge that is removably attached to the ink-jet recording apparatus, so as to make it easy to supplement the ink-jet recording apparatus with the ink. Such a cartridge type ink tank, hereinafter called the ink cartridge, is attached to the carriage in association with the recording head.
In order to notify the user of necessity to replace the ink cartridge, the ink-jet recording apparatus is provided with an ink sensor for detecting the residual amount of ink in the ink tank. For example, as disclosed in Japanese Laid-open Patent Application No. 7-117238, a photo sensor is used as the ink sensor, which consists of a light emitter emitting a light beam and a light receiver receiving a reflected light beam from the ink tank, to detect the residual amount of ink optically. The photo sensor is mounted to the carriage, with the light emitter and the light receiver placed in face to an ink detection zone that is located near a bottom of an ink chamber of the ink cartridge.
As the residual amount of ink in the ink chamber decreases, the liquid surface of the ink goes down toward the bottom of the ink chamber. Accordingly, when the liquid surface of the ink goes below the ink detection zone, the photo sensor detects that the ink does not exist in the ink detection zone, which is judged as an ink end of the ink cartridge.
However, as the ink cartridge moves back and forth together with the carriage, the ink waves in the ink chamber. Even while the liquid surface of the ink is lower than the detection zone when the ink cartridge stands still, the ink can flow into the ink detection zone, raising the level of the liquid surface of the ink up into the ink detection zone. Then, the ink sensor wrongly detects that the ink exits in the ink detection zone. The same detection error can occur in an ink cartridge that is unloaded before the ink is used up, and is reloaded, because such an ink cartridge can be shaken while it is carried about.